Fed cattle refers to cattle leaving a cattle feedlot, after fattening on a concentrated ration, that are ready to be sold to a packing plant for slaughter. Beef cattle are typically sold to packers at about 1,100 pounds, which yields a carcass weight of about 660 pounds.
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This article incorporates public domain material from Jasper Womach. Report for Congress: Agriculture: A Glossary of Terms, Programs, and Laws, 2005 Edition (PDF). Congressional Research Service.